Re: [PATCH 1/2] mm, memory_hotplug: do not fail offlining too early

[Michael Ellerman]

Michal Hocko <mhocko@xxxxxxxxxx> writes:

> On Tue 10-10-17 23:05:08, Michael Ellerman wrote:
>> Michal Hocko <mhocko@xxxxxxxxxx> writes:
>> 
>> > From: Michal Hocko <mhocko@xxxxxxxx>
>> >
>> > Memory offlining can fail just too eagerly under a heavy memory pressure.
>> >
>> > [ 5410.336792] page:ffffea22a646bd00 count:255 mapcount:252 mapping:ffff88ff926c9f38 index:0x3
>> > [ 5410.336809] flags: 0x9855fe40010048(uptodate|active|mappedtodisk)
>> > [ 5410.336811] page dumped because: isolation failed
>> > [ 5410.336813] page->mem_cgroup:ffff8801cd662000
>> > [ 5420.655030] memory offlining [mem 0x18b580000000-0x18b5ffffffff] failed
>> >
>> > Isolation has failed here because the page is not on LRU. Most probably
>> > because it was on the pcp LRU cache or it has been removed from the LRU
>> > already but it hasn't been freed yet. In both cases the page doesn't look
>> > non-migrable so retrying more makes sense.
>> 
>> This breaks offline for me.
>> 
>> Prior to this commit:
>>   /sys/devices/system/memory/memory0# time echo 0 > online
>>   -bash: echo: write error: Device or resource busy
>>   
>>   real	0m0.001s
>>   user	0m0.000s
>>   sys	0m0.001s
>> 
>> After:
>>   /sys/devices/system/memory/memory0# time echo 0 > online
>>   -bash: echo: write error: Device or resource busy
>>   
>>   real	2m0.009s
>>   user	0m0.000s
>>   sys	1m25.035s
>> 
>> 
>> There's no way that block can be removed, it contains the kernel text,
>> so it should instantly fail - which it used to.
>
> OK, that means that start_isolate_page_range should have failed but it
> hasn't for some reason. I strongly suspect has_unmovable_pages is doing
> something wrong. Is the kernel text marked somehow? E.g. PageReserved?

I'm not sure how the text is marked, will have to dig into that.

> In other words, does the diff below helps?

No that doesn't help.

> diff --git a/mm/page_alloc.c b/mm/page_alloc.c
> index 3badcedf96a7..00d042052501 100644
> --- a/mm/page_alloc.c
> +++ b/mm/page_alloc.c
> @@ -7368,6 +7368,9 @@ bool has_unmovable_pages(struct zone *zone, struct page *page, int count,
>  
>  		page = pfn_to_page(check);
>  
> +		if (PageReserved(page))
> +			return true;
> +
>  		/*
>  		 * Hugepages are not in LRU lists, but they're movable.
>  		 * We need not scan over tail pages bacause we don't
>
>
>> With commit 3aa2823fdf66 ("mm, memory_hotplug: remove timeout from
>> __offline_memory") also applied, it appears to just get stuck forever,
>> and I get lots of:
>> 
>>   [ 1232.112953] INFO: task kworker/3:0:4609 blocked for more than 120 seconds.
>>   [ 1232.113067]       Not tainted 4.14.0-rc4-gcc6-next-20171009-g49827b9 #1
>>   [ 1232.113183] "echo 0 > /proc/sys/kernel/hung_task_timeout_secs" disables this message.
>>   [ 1232.113319] kworker/3:0     D11984  4609      2 0x00000800
>>   [ 1232.113416] Workqueue: memcg_kmem_cache memcg_kmem_cache_create_func
>>   [ 1232.113531] Call Trace:
>>   [ 1232.113579] [c0000000fb2db7a0] [c0000000fb2db900] 0xc0000000fb2db900 (unreliable)
>>   [ 1232.113717] [c0000000fb2db970] [c00000000001c964] __switch_to+0x304/0x6e0
>>   [ 1232.113840] [c0000000fb2dba10] [c000000000a408c0] __schedule+0x2e0/0xa80
>>   [ 1232.113978] [c0000000fb2dbae0] [c000000000a410a8] schedule+0x48/0xc0
>>   [ 1232.114113] [c0000000fb2dbb10] [c000000000a44d88] rwsem_down_read_failed+0x128/0x1b0
>>   [ 1232.114269] [c0000000fb2dbb70] [c0000000001696a8] __percpu_down_read+0x108/0x110
>>   [ 1232.114426] [c0000000fb2dbba0] [c00000000032e498] get_online_mems+0x68/0x80
>>   [ 1232.115487] [c0000000fb2dbbc0] [c0000000002c82ec] memcg_create_kmem_cache+0x4c/0x190
>>   [ 1232.115651] [c0000000fb2dbc60] [c0000000003483b8] memcg_kmem_cache_create_func+0x38/0xf0
>>   [ 1232.115809] [c0000000fb2dbc90] [c000000000121594] process_one_work+0x2b4/0x590
>>   [ 1232.115964] [c0000000fb2dbd20] [c000000000121908] worker_thread+0x98/0x5d0
>>   [ 1232.116095] [c0000000fb2dbdc0] [c00000000012a134] kthread+0x164/0x1b0
>>   [ 1232.116229] [c0000000fb2dbe30] [c00000000000bae0] ret_from_kernel_thread+0x5c/0x7c
>
> I do not see how this is related to the offline path.

It's blocked doing get_online_mems(). So it's unrelated to the offline,
but it can't proceed until the offline finishes, which it never does,
IIUIC.

cheers